1. Field of the Invention
Aspects of the present invention relate to a method for manufacturing a thin film transistor with improved current characteristics and high electron mobility in which when an amorphous silicon thin film is crystallized into a polycrystalline silicon thin film by metal-induced crystallization, annealing conditions of the amorphous silicon thin film and the amount of a metal catalyst doped into the amorphous silicon thin film are optimized to reduce the regions of a metal silicide distributed at grain boundaries of the polycrystalline silicon thin film, and in which oxygen (O2) gas or water (H2O) vapor is supplied to form a passivation film on the surface of the polycrystalline silicon thin film.
2. Description of the Related Art
In general, polycrystalline silicon thin films are used in active matrix-liquid crystal displays, active matrix-organic lighting emitting diodes and solar cells. Typically, amorphous silicon thin films are crystallized into polycrystalline silicon thin films by crystallization. Various crystallization methods, such as laser crystallization methods and solid-phase crystallization (SPC) methods (e.g., high-temperature annealing and annealing using a metal catalyst), are currently used for the production of polycrystalline silicon thin films.
Surface silicon dangling bonds, internal grain boundaries and intragranular defects (such as twin defects, interstitial atoms, vacancies and sub-grain boundaries) are present in polycrystalline silicon thin films produced from amorphous silicon thin films, unlike in single-crystal silicon thin films. Such defects impede the migration of electrons and holes in polycrystalline silicon thin films to deteriorate the characteristics of devices (such as transistors) manufactured using the polycrystalline silicon thin films.
Further, a metal silicide (for example, NiSi2) is formed at grain boundaries of a polycrystalline silicon thin film during crystallization to impede the migration of electrons and holes, as shown in FIG. 1. Specifically, the metal silicide is present at grain boundaries of a channel region of a thin film transistor and acts as a defect deteriorating the characteristics (e.g., leakage current characteristics, electron mobility, and threshold voltage characteristics) of the device. Thus, the absence of metal silicide lines is needed to improve the leakage current characteristics of the thin film transistor.
Research on hydrogen passivation has been conducted to remove defects from polycrystalline silicon thin films. For example, a polycrystalline silicon thin film is passivated by hydrogen (H2) plasma passivation or annealing under a hydrogen atmosphere. The hydrogen added for the passivation of the polycrystalline silicon thin film is bonded to silicon dangling bonds of the polycrystalline silicon thin film. This bonding electrically neutralizes the polycrystalline silicon thin film and prevents defects from impeding the migration of electrons and holes in the polycrystalline silicon thin film.
However, a disadvantage of the hydrogen plasma passivation is that plasma may do damage to the surface of polycrystalline silicon thin films to degrade the characteristics of devices using the polycrystalline silicon thin films. The hydrogen atmosphere annealing may be performed by i) a method in which a silicon nitride thin film (SiNx) containing a large amount of hydrogen is formed on a polycrystalline silicon thin film and the hydrogen is diffused into the polycrystalline silicon thin film during annealing or ii) a method in which hydrogen is diffused into a polycrystalline silicon thin film during annealing under a hydrogen atmosphere. The method ii) has the disadvantage of long processing time resulting from slow hydrogen diffusion.
Further, when the temperature of a transistor increases, Si—H bonds formed by the passivation method can be easily split into hydrogen and silicon atoms, resulting in deterioration in the reliability of the transistor depending on the conditions of use.